1. Technical Field
The present invention relates to a pulse signal generating device that generates a pulse with a restored period by detecting period abnormalities in an output pulse of an encoder, a transport device, an image forming apparatus, and a pulse signal generating method.
2. Related Art
A known image forming apparatus, such as a printer, has a configuration in which a recording head performs printing on paper (target) transported in a transport direction. In this case, it is necessary to discharge ink droplets at proper timing according to the paper transport position. For this reason, a print reference signal is generated in synchronization with the transport speed of paper on the basis of an output signal (pulse signal), which is output from an encoder in synchronization with the paper transport speed, and the discharge timing is controlled on the basis of the print reference signal.
For example, JP-A-11-245383 discloses a printer (image forming apparatus) that uses a transport belt as a paper transport unit. A mark for detecting the speed and position is set on the transport belt, and a character or an image is printed on the paper by reading the mark with an encoder and discharging ink on the basis of the encoder signal.
As disclosed in JP-A-11-245383, in a printer using a paper transport unit such as a transport belt, ink droplets are discharged for every desired pitch by detecting the amount of movement of recording paper from the amount of movement of a belt in a condition where the amount of movement of recording paper is assumed to be equal to the amount of movement of the belt. Since discharge of ink droplets is performed in synchronization with an encoder signal having pulses with the same distance as a print pitch, high-quality printing capable of suppressing the positional deviation of landing ink droplets becomes possible even if there is a speed variation in the transport unit. However, since encoder signals need to be output continuously at the same pitch in the method disclosed in JP-A-11-245383, the following problems occur.
If the circumferential length of the recording paper transport unit (transport belt) is not integral multiples of the print pitch, a discontinuous portion occurs in an output signal of a linear encoder, which causes image deterioration. Moreover, when ink mist or paper particles adhere to the linear encoder disposed on the transport belt or the linear encoder is damaged, lack of a pulse occurs in the encoder signal. Also in this case, image deterioration occurs.
A device for solving such a problem is disclosed in JP-A-2005-91943 and JP-A-2005-350195 (for example, paragraphs [0041] to [0053] in the specification and FIGS. 6 to 11), for example. JP-A-2005-91943 discloses a device that uses two sensors while switching the sensors at the home sensor position. A phase difference between signals at the time of sensor switching is detected by pre-driving.
JP-A-2005-350195 discloses a configuration where two sensors for detecting a mark are provided such that positions of objects to be detected are different and an output signal, which is used for motor control for controlling the transport speed of a belt constantly, is switched to an output signal of the other sensor when a discontinuous portion of a mark, such as a joint of the transport belt, or lack of a pulse caused by dust or damage is detected on the basis of an output signal of one sensor. Moreover, in order to suppress the phase difference between signals at the time of sensor switching, a configuration where an interpolation processing portion generates a high-resolution signal by dividing a measured period or using the same clock so that a signal error (phase difference) is reduced is adopted. Moreover, JP-A-2005-91943 and JP-A-2005-350195 disclose a method of counting a period of a sensor signal (encoder signal) with a base clock and determining that there is a discontinuous portion (lack of a pulse of an encoder signal) of a mark when the pulse period of the sensor signal reaches a fixed threshold value or more.
However, the technique disclosed in JP-A-2005-91943 is effective only in a place set beforehand as a joint, and joint position detection using a home sensor is needed. In addition, there was a problem that it was not possible to cope with sudden missing.
Furthermore, according to the device disclosed in JP-A-2005-350195, a phase difference frequently occurs due to phase matching using period division. In addition, there is also a problem that pulse missing detection is delayed. For this reason, since a pulse period become long in a portion where missing occurs, a period from an interpolation pulse to the next normal pulse output becomes short. In order to prevent this, if a threshold value is set small, there was a possibility of erroneous detection of pulse missing even when the normal pulse generation time was overdue due to speed variation.